Within-Flock Population Dynamics of Dichelobacter nodosus

Abstract

Footrot causes 70-90% of lameness in sheep in Great Britain. With approximately 5% of 18 million adult sheep lame at any one time, it costs the UK sheep industry £24-84 million per year. The Gram-negative anaerobe Dichelobacter nodosus is the causative agent, with disease severity influenced by bacterial load, virulence, and climate. The aim of the current study was to characterize strains of D. nodosus isolated by culture of swabs from healthy and diseased feet of 99 ewes kept as a closed flock over a 10-month period and investigate persistence and transmission of strains within feet, sheep, and the flock. Overall 268 isolates were characterized into strains by serogroup, proline-glycine repeat (pgr) status, and multi-locus variable number tandem repeat analysis (MLVA). The culture collection contained 87 unique MLVA profiles and two major MLVA complexes that persisted over time. A subset of 189 isolates tested for the virulence marker aprV2 were all positive. The two MLVA complexes (76 and 114) comprised 62 and 22 MLVA types and 237 and 28 isolates, respectively. Serogroups B, and I, and pgrB were associated with MLVA complex 76, whereas serogroups D and H were associated with MLVA complex 114. We conclude that within-flock D. nodosus evolution appeared to be driven by clonal diversification. There was no association (P > 0.05) between serogroup, pgr, or MLVA type and disease state of feet. Strains of D. nodosus clustered within sheep and were transmitted between ewes over time. D. nodosus was isolated at more than one time point from 21 feet, including 5 feet where the same strain was isolated on two occasions at an interval of 1-33 weeks. Collectively, our results indicate that D. nodosus strains persisted in the flock, spread between sheep, and possibly persisted on feet over time.

Keywords: Dichelobacter nodosus; MLVA; footrot; persistence; strain variation.

Figure 1

Minimum-spanning trees of MLVA profiles (A), supplemented with data on serogroups (B), pgr status (C), and disease state (D) of Dichelobacter nodosus cultures isolated from a flock of 99 ewes repeatedly sampled over 10 months. The seven most frequently isolated MLVA types are indicated by number in panel (A); predicted founder strains have a black border and subgroup founder strains a gray border, and circle size is proportional to numbers of isolates. Colors indicate (A) MLVA complexes: MC76 (orange); MC114 (blue); minor group I (green); and singleton (purple). Lines between MLVA profiles in the same MLVA complex indicate single locus variants, and between isolates in different MLVA complexes indicate double locus variants. (B) Serogroup: B (orange), H (blue), I (green), and D (purple). (C) pgr status: A (orange) and B (blue). (D) Foot disease state: healthy (orange), mild interdigital dermatitis (ID) (blue), severe ID (green), and severe footrot including lesions (purple).